The invention relates to a frequency discriminator for a coherent optical transmission system, comprising an input for supplying a FSK input signal having the frequencies F(1) and F(2). Coherent optical transmission systems are attractive as future systems with wide repeater spacing and large information capacity. The coherent optical transmission systems, wherein phase or frequency modulation and optical heterodyne or homodyne detection schemes are used offer the great advantage that the power of the receiver can be much smaller than is customary in the conventional PCM intensity-modulated systems.
Such a coherent optical transmission system is for example described in IEEE Journal of Quantum Electronics, Vol. QE.17, No. 6, June 1981, pp 935-940 and in IEEE Journal of Quantum Electronics, Vol. QE.19, No. 2, February 1983, pp 180-192. In the known coherent optical transmission systems using FSK, a detector commonly referred to as coincidence detector is used as a frequency discriminator, which detector is formed by means of a delay line and a double-balanced mixer, for example a bridge configuration of diodes. However, this known frequency discriminator has the disadvantage of not being stable and flexible anymore with frequencies exceeding 1 GHz. A further disadvantage is that with these high frequencies usually two separate discriminators are required, that is to say, one for automatic frequency control (AFC) and one for FSK detection. The known frequency discriminator further has the disadvantage because of its composition that it is hard to integrate in a semiconductor body.
The invention has for its object to provide measures for removing the aforesaid disadvantages and is characterized in that the input is connected to the two inputs of a first full-wave rectifier via a first differential stage, each one of the full-wave rectifier inputs being connected to a short-circuited transmission line having a length of approximately .lambda.(1)/4, with .lambda.(1) representing the wavelength going with the frequency F(1), the short-circuited ends of the transmission lines being connected to a point of constant potential via a capacitor, the full-wave rectifier-output being connected to an open transmission line having a length of approximately .lambda.(1)/8, and in that the input of the discriminator is connected to the two inputs of a second full-wave rectifier via a second differential stage, each one of the inputs of the second full-wave rectifier being connected to a short-circuited transmission line having a length of approximately .lambda.(2)/4, with .lambda.(2) representing the wavelength going with the frequency F(2), the short-circuited ends of the transmission lines being connected to a point of constant potential via a capacitor, the output of the second full-wave rectifier being connected to an open transmission line having a length of approximately .lambda.(2)/ 8.